1. Field of the Invention
The present invention relates to a semiconductor memory device and a manufacturing method of a semiconductor memory device, e.g., a fin-type FBC (Floating Body Cell) memory.
2. Related Art
Development of technology for manufacturing full-depletion FBC (“FD-FBCs”) on an SOI substrate has been underway. Each of the FD-FBCs includes a back gate to increase the signal difference (potential difference) between data “0” and “1”. A thickness of a buried oxide film (BOX) layer is preferably smaller (for example, 25 nm or less) for the back gate to facilitate controlling body bias.
Nevertheless, the thickness of the BOX film is generally about 150 nm and it is technically difficult to make the thickness of the BOX layer equal to or smaller than 50 nm. To tackle the problem, a technique using a FinFET as an FBC has been developed. If the FinFET is used as the FBC, a thin gate insulating film can be formed on each side surface of a Fin (i.e., the FinFET). Therefore, the signal difference (potential difference) between the data “0” and the data “1” can be increased. In this case, the thickness of the BOX layer has no effect on characteristics of the FBC.
In a conventional FBC constituted by the FinFET, a silicon part provided between two front gates is present. Because this silicon part is not electrically activated, the silicon part does not contribute to FBC operations. An area of the silicon part occupies 30% or more of that of a memory region, so that the silicon part obstructs downsizing of a semiconductor memory device.